The present invention generally relates to a developing apparatus for use in electrophotography, and more particularly, to a developing apparatus which is arranged to develop an electrostatic latent image to be formed in electrophotography, electrostatic recording or the like through employment of one or mono-component developer, thereby to obtain a visible image.
Conventionally, there have been proposed many methods for the image formation based on the electrophotography as disclosed, for example, in U.S. Pat. No. 2297691 and Japanese Patent Publication Tokkosho No. 43-24748. Generally, such method is so arranged that, by forming an electrostatic latent image on a photosensitive member or photoreceptor having a photoconductive layer formed thereon, such latent image is subsequently developed through employment of toner, and a toner image thus obtained is transferred onto a transfer material such as a copy paper sheet or the like for fixing.
Meanwhile, various methods have also been conventionally proposed for visualizing the electrostatic latent image by the use of toner, among which particularly, the magnetic brush method, cascade method, and liquid developing method etc. which utilize the developing material mainly composed of toner and carrier are widely put into practical applications. Although favorable images may be obtained comparatively stably by any of the above methods, there are common problems related to two or dual-component developing material such as deterioration of toner, and variation of mixing ratio of toner to carrier, etc.
In order to overcome such problems, there have also been proposed various methods using the mono-component developing material composed only of toner.
In one of such mono-component developing methods as disclosed for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 54-43036, it is so arranged that magnetic toner is triboelectrically charged, and applied over a developing sleeve in a very thin layer of toner, which is brought close to an electrostatic latent image so as to confront said latent image without contact therewith, thereby to effect developing under the action of the magnetic field.
By the above known method, favorable images may be obtained by the facts that a sufficient triboelectric charging is available, since the developing sleeve and the toner contact each other more closely by applying the magnetic toner onto the developing sleeve in the extremely thin layer, that the developing sleeve and toner are fully rubbed against each other, while coagulation among toner particles is intended to be solved by relatively moving the magnet and toner, with the toner supported by the magnetic force, and that ground fogging is prevented through development by causing the toner supported by the magnetic force to confront the electrostatic latent image for the developing, without contact therewith.
However, in the conventional practice as described above, the applicability of toner over the surface of the developing sleeve is readily affected depending on environmental conditions, and the toner is not sufficiently stirred, especially with a very small amount of displacement in the widthwise direction on the sleeve, and thus, various problems are brought about thereby.
For example, if fluidity of toner is lowered at high humidity or high temperature, coagulation of the toner can not be fully solved by the magnetic force, thus resulting in the lowering of the image quality and image density.
Meanwhile, during a period at low humidity, since the toner that is highly triboelectrically charged strongly adheres onto the developing sleeve, the toner layer over the sleeve becomes uneven when the sleeve is repeatedly rotated, thus also adversely affecting the image quality.
Moreover, when many sheets of original documents having image portions deviated in position, are copied or printed continuously, at portions not used at all for the developing, image density is undesirably lowered when the copying or printing is subsequently effected.
Furthermore, there are such problems that cost for the apparatus is increased by the employment of the magnet, and that clear color toner can not be used due to adoption of the magnetic toner.
Accordingly, as a method for solving the problem as described above, there has also been conventionally proposed a developing method which employs a non-magnetic mono-component toner.
The known developing method as described above is so arranged that, for example, as shown in FIG. 5, restricting members 81 are pressed against a cylindrical toner holding member 82 made of an elastic roller, and with toner T in a tank C being supplied to a nip portion N between said restricting members 81 and the toner holding member 82, said toner holding member 82 is driven for rotation in a direction indicated by an arrow A, whereby a thin and uniform toner layer is formed on the toner holding member 82, while the toner T is being triboelectrically charged by being rubbed at the nip portion N, and said toner layer is transported onto a photoreceptor PC, and by impressing a developing bias V onto the toner holding member 82, the electrostatic latent image on the photoreceptor PC is developed into a visible image.
In the above conventional developing method, since the toner T is charged through friction with respect to the toner holding member 82 and the restricting members 81, triboelectric charging characteristics of the toner holding member 82 and the restricting members 81 become important. If the toner holding member 82 is made of a metallic material, since its width in the frictional charging row is small and range (width for selection of triboelectric charging polarity thereof is also small, its difference in polarity with respect to the toner T can not be increased, and therefore, the toner has a wide charging distribution, while toner in the opposite polarity is produced, with consequent deterioration in the toner charging quality, and thus, fogging is formed in the copy or print images so as not to provide clear and definite images.
Additionally, in the above known practice, there has been proposed means for improvement such as plating of various kinds of metal on the surface of the toner holding member 82. By such a practice, however, not only cost is increased by that extent, but the range of selection for the triboelectrical charging polarity is undesirably narrowed.
Furthermore, for the developing, when a developing bias which will form alternating field at the developing region is impressed, the toner holding member 82 and the photoreceptor PC may be in the state of non-contact, but in the case where the electric field at the developing region is of a d.c. field, the toner holding member 82 and the photoreceptor PC are generally disposed in the state of contact. However, when they are held in contact with each other as described above, there are problems related to cylindricity and eccentricity of the toner holding member 82 or the photoreceptor PC, with large variations in the state of contact at the developing region, thus adversely affecting the image quality.
For solving the problem as described above, there has also been proposed a developing method in which the disadvantage inherent in the contact developing method as described above has been eliminated, for example, in U.S. Pat. No. 4,791,882.
More specifically, as shown in FIG. 6, the developing apparatus for effecting the above known developing method includes a driving roller 1 to be driven for rotation in a direction of an arrow b by a driving means (not shown), a cylindrical thin film developing sleeve 2 having a circumferential length longer than that of the driving roller 1 and loosely mounted around the driving roller 1, a guide member 3 which contacts opposite end portions of the driving roller 1 through said developing sleeve 2 so as to form a space S between the driving roller 1 and the developing sleeve 2 at a position where said driving roller 1 confronts an electrostatic latent image holding member PC arranged t be rotated in a direction indicated by an arrow a, and also to bring an outer surface of the developing sleeve 2 covering said space S into close contact with an outer peripheral face of said electrostatic latent image holding member PC at a developing region X, a blade or toner restricting member 4 pressed against the outer surface of said developing sleeve 2 contacting the driving roller 1 for forming a thin layer of charged toner over the outer peripheral face of said developing sleeve 2 rotating together with said driving roller 1, and a developing tank C accommodating therein said driving roller 1, developing sleeve 2, guide member 3, and toner restricting member 4 and containing therein a certain amount of toner T, with an agitator 5 being further provided within said developing tank C for rotation in a direction indicated by an arrow for prevention of blocking of the toner T.
However, in the various developing methods by the mono-component developing material as described so far, it is so arranged that the toner particles are triboelectrically charged by friction, etc. with respect to the toner holding member and the developing sleeve or the toner restricting member, and the developing is effected by contacting said tone particles with the electrostatic latent image holding member or in the state of non-contact of the toner particles therewith. In these methods, there is such a problem that the number of times for contacts between the toner particles and the tribo-electrical charging member is small, and the triboelectric charging tends to become insufficient. Moreover, since the restricting member is held in contact with the toner holding member under a pressure larger than a certain degree, there are also such disadvantages that the toner in a fused state adheres to the restricting member and the surface of the toner holding member is abraded, and/or filming of the toner takes place on the toner holding member, thus not providing a sufficient toner charging amount, without formation of a uniform thin layer of the toner as desired.
Accordingly, in the case where the electrical latent image is to be developed through utilization of the triboelectric charging by the use of the mono-component insulative toner, the surface characteristic of the tribelectric charging member such as the toner holding member is extremely important, and as one factor, the frictional force and affinity between the toner and the triboelectric charging member determines the stable charging characteristic of toner. In other words, if the coefficient of friction on the surface of the triboelectric charging member is too small, toner tends to slip at the toner restricting portion, resulting in a thin and uneven application, while on the contrary, if such coefficient of friction is too large, the applied toner layer becomes excessively thick, thus making it difficult to impart a sufficient triboelectric charge to the toner.
In connection with the above, although it is considered to employ a resin material for the triboelectric charging member from the aspects of the triboelectric charging characteristics, image quality and cost, etc., there still remain such problems as the tendency to fusion of toner onto the surface of the toner restricting member or toner holding member, and also, abrasion on the surface of the toner holding member.